Hydraulic constant speed control system for parallel-connected hydraulic motors



Feb. 11, 1947. J M LLER ET AL 2,415,603

HYDRAULIC COI ISTANT SPEED CONTROL SYSTEM FO PARALLEL-CONNECTED HYDRAULIC MOTORS Filed Aug. 26, 1943 2 Sheets-Sheet 1 i a INVENTO s JOHAN A.MOLL R WARREN RZTOCRER,

' T MXWZM ATTORNEY$ Feb. 11, 1947. MULLER ET AL 2,415,603

HYDRAULIC CONSTANT SPEED CONTROL SYSTEM FOR PARALLEL-CONNECTED HYDRAULIC MOTORS Filed Aug. 26, 1943 2 Sheets-Sheet 2 INVENTORS Jon/m A. MULLER,

R E K.TU KER,' WA R N I ATTO R N EYS Patented Feb. 11, 1947 I HYDRAULIC CONSTANT SPEED CONTROL SYSTEM FOR PARALLEL-CONNECTED H Y- .DRAULIC MOTORS Johan A. Muller and Warren R. Tucker, Dayton, Ohio, assignors to The Hydraulic Development Cbrp., 1110., Wilmington, DeL, a corporation of Delaware Application August 26, 1943, Serial No. 500,068

9 Claims. 1

This invention relates to hydraulic machinery and, in particular, to a control system for a'plurality of hydraulic motors, i. e.,v for two motors or for any given number of motors, especially when said motors are actuated from a common fluid source.

When a plurality cf'hydraulic motors is sup- .plied with pressure fluid from a common fluid source, the motor encountering the lowest resistance tends to run ahead of the other motor or motors and said motor which encounters the It is another object of this invention to pro-' vide a hydraulic system including a plurality of fluid operable motors supplied with pressure fluid from a common fluid source, in which a change in resistance encountered by one motor will have practically no effect on the speed of one or more of the other motors of said system.

It is anotherobject of this invention to provide a hydraulic system including a plurality of fluid operable motors and in which two or more motors may be supplied with pressure fluid from a common fluid source, and in which control means is provided adapted automatically to maintain the speed of each individual motor substantially constant.

3 Another object of this inventionconsists in the provision of a hydraulic system including two or 'more fiuid operable motors supplied with'pressure fluid from a fluid source common to all of said motors, in which the said control means is provided, in response to a decrease or increase in resistance encountered by onev motor to bring about a corresponding change in the supply of pressure fluid to said motor to restore the speed of said motor. while the speed of the other'motor or motors is held substantially constant.

It is a still further object to provide a hydraulic system including two or more fluid operable motors supplied with pressure fluid from one and the same fluid source, in which means is provided for maintaining a desired back pressure at said motors, andin which, responsive to an increase variable chokevalve so as to cause the-latter to create back pressure on its motor. In addi' or decrease in resistance encountered by one or I more of said motors, the supply of actuating fluid to said motor or motors is decreased or increased of example, an

2 respectively, while the speed of the other motor or motors is maintained substantially constant.

Another object of the invention consists in the provision of a hydraulic system including two or more operable motors supplied with pressure fluid from one and the same fluid source, in which the speed of one or more motors may selectively be varied without substantially afiecting the speed of the other motor or motors. 1

Another object of the invention is the provision of a hydraulic system employing two or more motors which are operated from a, single fluid source, and in which the motors may be either of the rotary or reciprocating type, or a combination ofsaid types, and which motors need not have any relationship to each other as far as size or type are concerned.

A still further object of the invention consists in the provision of a hydraulic system including a variable delivery pump which drives .two or more fluid motors and in which there is a control cylinder provided for each fluid motor which control cylinder is hydraulically connected with means which controls the amount of fluid delivered by the pump.

' These and other objects and advantages of the invention will appear more clearly from the iollowing specification in connection with the accompanying drawings which illustrate, by way embodiment of the new control system. I

The control system shown in the drawin s comprises in general three hydraulic motors supplied with pressure fluid from a common variable delivery source, each motor being hydraulically connected with a pressure responsive variable choke valve, which in its turn is connected with means for creating back pressure in said motors, and is also in fluid communication with a control mechanism associated with the fluid source feeding said motors. The. said control system is pressure responsive and adapted to vary the supply of pressure fluid to the motors.

A variation in resistance encountered by one.

of said motors reacts on its pressure responsive tlon thereto, the delivery of fluid from said fluid source to said motors is changed. Then anew pressure condition. prevails because the pressure! created by the fluid source stands on all motors at a value determined by the greater motor load.

Figure 2, a vertical section of a modified form a of valve with conduits connected thereto, and

Figure 3 is a plan view of a modifledform of apparatus wherein it is possible to determine the retraction speed as well as the forward speed of each of the controlled motors.

Referring now to the drawings in detail the structure shown in Figure 1 comprises a pump I of the variable delivery type. which may be a radial piston or radial vane pump. The flow control member of the pump I has connected thereto a control rod 2 with a piston 3 which latter is reciprocably mounted in a servomotor or control cylinder 4. The flow control member of the pump I is continuously urged by means of a spring in tors I3, I4 and I5. These motors I3, I4 and I5 may be of any suitable size, and may be either I of the rotary or reciprocating type. It is not necessary that these motors bear any relationship to each other so far as size or type is concerned, i. e., any one of them may be of the rotary type and another of them may be of the reciprocating type, and each of them may vary as to size. The branch lines II), II and I2 connect to the inlet side of these hydraulic motors. Connected to the exhaust side of said motors are a plurality of conduits I6, I! and I8, which latter respec- I a casing 6 in a direction to move'the pump into tively lead to annular recesses I9, 29 and 2i in the valve casings 22, 23 and 24 of the pressure responsive variable choke valves generally designated 25, 25 and 21.

'Reciprocably mounted in the valve casings are 'valve members 28, 29 and 30. The valve member 28 is provided with interconnected piston portions 3 I, 32, the valve member 29 is provided with similar piston portions 33, 34 and valve member with similar piston portions 35 and 35.

The conduits 31, 38 and 39 connect the spaces between each pair of pistons to choke valves 40, H and 42. Bypass lines 43, 44 and 45 are connectedintermediate their ends into conduits 31, 7

3B and 39. The left hand end of each bypass is connected into each valve casing near the left hand end of the cylinder bore of that casing so as to be uncovered during most of. the movei ments of the left hand piston. The right hand end of the bypass is connected into the cylinder bore so as to be ordinarily covered by the right hand piston. Located in each bypass between the and 48. These check valves may open so as to bypass fluid only in the direction indicated by conduits 31, 38 and 39 are check valves 46, 41

the arrows.

Also connected into the valve casings 22, 23 and 24 are conduits 49, 5B and 5|, respectively, which lead from the right hand end of the valve bores to a line 52 and which leads into the control cylinder 4.

. All of the conduits 31, 33 and 39 are connected to a return conduit 53 which leads into the tank 8.

In order to allow the pump I to shift through its complete stroke without having to rely on the displacement of fluid from the valve casings,

an extra line is provided to the control cylinder 4 consisting of short conduits 54, 55 and 55, connected into the midpoints of the valves 22, 23

and 24 respectively, and each having a check a valve 51, 58 and 59, respectively. These check valves will only open in the direction indicated by the arrows. Each of these short conduits connects into the line 50, and interposed between the fluid which flows out of the short conduits 54, 55 and the control cylinder 4 is a choke valve In Figure 2 there has been illustrated a modification of the pressure responsive variable choke valve shown in Figure 1. The pressure responsive variable choke valve shown in Figurez is designated generally by 52 and includes casing 53. Located within the casing is a valve member 34 having piston portions 65 and 66. The piston 35 is provided with its own bypass which consists of a plurality of passageways 61 which extend from one side of the piston to the other. The interior of the casing is provided with an annular recess 68. i

Leading into'the cylinder between the pistons 65 and 65 and which corresponds to the short conduits 54,55 and 56, is a conduit 59 which is provided with a check valve 10, which latter may only open in the direction of the arrow. Conduit 69 connects with the conduit II which latter corresponds to the conduit 59, Fig. 1, and which may lead back through a suitable choke valve I9, corresponding to SI, to the control cylinder 4. Connected to the interior of the casing 53 adjacent the right hand thereof is a short conduit 12 which is connected into the return conduit I3. The conduit I2 of Fig. 2 corresponds to conduit 49 of Fig. l and conduit 13 corresponds to conduit 52 of Fig. 1, and said conduit I3 connects into'the control cylinder 4.

There is a bypass I4 and check valve I5 which corresponds to the bypass located to the right in Fig. 1 and to the check valve 46. The casing also has an extension I6 in which is located choke valve I1 and a pipe I8 which connects to the tank 3 and which corresponds to the conduit 53 in Fig. 1. V

The valve I9 indicates a manually adjustable choke valve, the function of which is to replace choke valve 6|. It will be noted that it is in the.

same position in the circuit, namely between the right hand and the central chambers of the valve Operation It may be assumed that the pump I has been started, and that fluid is flowing from the pump I through conduit 9 into branches I9, II and l2 to the hydraulic motors I3, I4 and 15. Exhaust fluid from each of the hydraulic motors leads to the interior'of the valve casings 22, 23 and 24. The pump control cylinder 4 is assumed to be of suflicient size to shift the pump to neutral or-idling position at a predetermined low pressure, of, say, about 50 pounds per square inch. Inasmuch as the return line 52 from the control cylinder 4 is connected to the valve casings 22', 23 and 24 and into the space between the right hand casing wall and the right hand face of each piston 3|, 33 and 35, the pressure within fluidwithin the valve member. 'the volume of fluid within the control cylinder against the piston 3 within the control cylinder 4. In order to balance the valve members pressure must be the same in the spaces between the left hand ends of the valve casings and the left hand ends of the piston portions 32, 34 and 38, and the pressure between each pair of pistons such as 3|, 32 must also be the same inasmuch as the conduits 31, 43; 38, 44, and 39, 45 are interconnected. This is also true of the construction disclosed in Figure 2 as the passageways 6! connect the opposite ends of the piston 55.

The chokes 40, 4| and 42 will always maintain a back pressure in the valve casings 22, 23 and 24.

The spring 5 on the pump tends to force the pump into full stroke position. The control piston 4, when it is moved toward the right, forces fluid through the line 52 and conduits 49, 50 and 5|, thereby moving the valve members 28, 29 and members and other factors such as the rate of flow, etc, 7

Assuming that the discharge ofthe pump flows to the hydraulic motors and that all of the chokes have been set at the required speed, if there is no work resistance met by the severalmotors they tend to speed up. The speed up of the motors will increase the flow from the conduits I6, I?

and I8 into the annular recesses i9, 20 and 2| and from the spaces between each pair of pistons of each valve member and into the pipes 31, 38 and 39 to the chokes 48, 4| and 42. This results in a'building up of pressure ahead of the choke, and this pressure will act on the left hand 6 closed and choke 4| is open to substantially its maximum extent.

As the choke 4| is opened, the pressure between the pistons 33 and 34 and on the left hand side of piston 34 drops. This will cause the valve 5 member 29 to move to the left and it may even side of each piston 32, 34 and 36 thereby moving the valve members, to the right and partially the output of the pump. Fluid'will also flow from the spaces between the pistons of each valve member through the conduits 54, and 56, check valves 51, 58 and 53, line 88, choke 5| to the right hand side of piston 3.

It will be understood that the line 50, together with its conduits 54, '55 and 56, as well as the bypasses 43, 44 and 45 permit the pump to go through its complete stroke, 1. e. to shift from full delivery position to nodelivery position, without having to .rely solely on the displacement of In other Words need not be the same as thecombined volumes of fluid in the valve casings 22, 23 and 24.

The purpose of the individual chokes 48, 4| and 42 is to be able to manually adjust the speed of each hydraulic motor which it controls. Assuming motors l3, I4 and I5 are running at .the same speed and'lt is desired to run motor M at its maximum speed and to decrease the speed of motors l3 and I5, to accomplish this result chokes 40 and 42 are moved so as to be more nearly move in this direction to such an extent as to uncover the right hand connection of the bypass line 44 which connects through the check valve '47 to the interior of the valve casing adjacent the right hand end thereof.' Fluid will simultaneously flow through the conduit 38, return conduit 53 to the tank Band pressure will be relieved in the line 52 through the connection 58 into the valve casing and through the bypass, and'out the pipe 38 and conduit '53 to the tank 8. This will momentarily drop pressure in the cylinder 4 and, through the action of the spring 5', cause the pump to move to increased stroke position.

As the velocity of the motor l4 increases the back pressure also increases to a certain extent and will cause the valve member 29 to move to th right so as to control the motor l4.

Valve members 28 and 38 will also find their equilibrium .at substantially the same time as valve member 29.

.It will be apparent that the bypass connection at the right hand end of the valve 23 is effective to permit the spring 5 to .move the pumpon full stroke, if necessary, the fluid thus displaced from the cylinder 4 passing through the conduit 5|, the conduit 58, the check valve 41, conduit 38, choke valve 4| and conduit 53 to the tank 8. Also, the fluid communication from the cylinder 4 through the choke valve GI and the choke valves 51, 58 and 59 to the valves 22, 23, and 24 is effective to permit the pump to move substan-' tially to neutral, if necessary, regardless of the fluid displaced from the said valves, providing the pressure at the center of .any one of the valve is great enough.

The volume of oil displaced by the control system is therefore not dependent upon the volume of oildisplaced by the valve members.

It will therefore be understood that'the structure disclosed herein provides a means whereby the speed of two or more hydraulic motors which are driven from a single pressure source may be controlled. i

Thisis achieved by the provision of a pressure responsive'variablechoke valve attached to the exhaust side of each hydraulic motor, the value of which is determined by the pressure drop across an adjustable choke, one of which is provided with each pressure responsive variable choke valve. The valve member of the pressure responsive variable choke valve will move to a position such that pressure drop across it will hold back pressure on the hydraulic motor which it controls and thereby maintain the speed of the hydraulic motor constant. The speed at which each hydraulic motor is to be run may be adjusted by means of the manually controlled adjustable choke. Therefore the individual speed of each .of two or more motors driven from the same pressure source may beaccurately controlledso that the speed of each motor will stay at a predetermined value even under great vari-' ations in loads. I

It-should further be noted that the yariable delivery source of fluid pressure is variable only as necessary to correct momentary changes in motor speeds and will always return substantially to its original position regardless of load changes in position of the pressure responsive variable choke valve. The delivery of the pump is changed permanently only by a change in setting of one or more of the manually adjustable chokes.

, in the control system which affects the pressure responsive variable 'choke valve of the varying motor, to restore its speed, affects the pump stroke to help restore the speed of this motor, and

affects the pressure responsive variable chokev valves of the other motors to maintain their speed by opposing the change in pump delivery. For example, if motor I should slow down the action is as follows:

a. The pressure drops in the groove 2| because of reduced flow.

b. The pressure drops on the leftside of valv member 90, due to (a) and the valve shifts to the left decreasing the restriction at 2I.

c. The movement of the piston 30 to the left allows fluid to flow in conduit 52 putting the pump on increased stroke.

d; The valve members 28 and 29 shift to the right for the same reason that the pump stroke is influenced, namely the removal of fluid from conduit 52 by movement of the valve member ,tor is absolutely independent of the others.

change of load its speed is momentarily aflected but due to the foregoing operation its speed will be restored to normal. Should it be desired, however, to increase or decrease the speed of any one of the motors, this can be accomplished by the manual operation -of its corresponding manually operated choke valve 40, M or 42.

The second sheet of drawings shows an embodiment of this invention as applied to reciprocating motors. In this view, means are provided whereby a plurality of reciprocating motors may be controlled by the mechanism of this invention in such a manner that the operation of one mo- In fact, as stated before, the motors need not be reciprocating motors but could be rotary motors or any arrangement of rotary and recip-' rocating motors.

Such a situation might arise in the event of several hydraulically actuated machines, individually and separately operated, being supplied with pressure fluid from a common source.

This View, Figure 3, also shows a means for extending the control to the retraction stroke of a reciprocating motor.

Thisis accomplished as follows:

The motors 90 and 9| are supplied with pressure fluid by means of the conduit 9 from the pump I, the fluid passing through conduits 92 30, increasing the restrictions at I9 and and thereby neutralizing the effect of the increased pump delivery on the motors l3 and I4.

It is apparent that the slowing down of motor I5 has caused changes in the system tending to correct the speed thereof, and that these changes also act to cause motors I3 and I4 to be unresponsive to the correcting influences. Now all of this is temporary and after a short time all of the original speed conditions are restored and the pump stroke returns to normal. The only permanent change inthe system is a new location ofthe valve member to compensate for the changed load on motor I5.

.Ihe movable element of each pressure respon- I sive variable choke-valve, i. e., member! 28, 29

variable choke valves in order to prevent the pump I from fluctuating too much whenever the load changes in one or more of the,hydraulic motors I3, I4 or l5. This valve 6| also helps to establish a pressure differential between the two ends of one of the pressure responsive vari- .able choke valves during a change of condition of its hydraulic motor, 1. e., when the load either increases or decreases in the corresponding motor of the corresponding pressure responsive variable choke valve. I

The partial closing of any one of the choke valves 40, 4! or "will slow down the operation of its corresponding fluid motor I3, I4 or 5. Should one of these choke valves be partially closed the speed of that motor will be slowed down but the speed of the other motors will be Innafiected.

To sum up: Assuming all of the motors are operating under the same conditions of speed -'and load, should one of the motors encounter a and 93 into pilot operated, four way valves 94 and 95 and thence through conduits 96, 91, 98 and 99 into the motors 90, 9| where the fluid pressure isconverted into mechanical force by means of the working urfaces of the pistons.

Fluid is exhausted from the motors through the four way valves and into conduits I00, IOI,

whence it passes into the balancing valves, at

. I02, I03. These valves are identical with those for the purpose of interconnecting the conduits I04 and I05 with either conduits I00 and H0 respectively or with conduits I09 and III respectively.

Each of the conduits I08, I09, IIO, III, has a choke valve I I2, H3, H4, H5 therein, the functions of which are identical to the functions of the choke valves 40, 4 I, 42 in Figure 2.

A pilot pump I02 is provided for the purpose of supplying pressure fluid for the purpose of shifting the valves 94, 95, I06 and I01. Pilot valves I2I and I22 are provided for thepurpose of selecting the desired position of valves 94, 95, I06 and I01. Pilot fluid is supplied to the pilot valves through conduits I23 and I24- and conducted therefrom to the controlled valves by means of conduits I25, I26, I21 and I28. Exhaust fluid is conducted from the pilot valves to the reservoir by means of a conduit I29.

Conduits and valves interconnect the pressure responsive variable choke valves and the pump control cylinder in a manner identical to that shown in Figure 1.

Operation ofthe structure shown in Figure 3:

In Figure 3 the motor is on the retraction stroke and the motor 9| is on the forward stroke. Motor 90 has been placed on the retraction stroke by the operator ofthe machine, of which it is a part, byhis positioning the pilot valve id ceiving fluid from said source; a variable restricting valve connected with each of said motor outlets for passing the entire discharge of the assothe retraction speed as well as the forward speed of the controlled motors.

It is obvious that the choke valve in conduit l i2 must be different than that in conduit H3 since one must pass the fluid from the main chamber and the other must pass only the fluid from the retraction chamber.

While only two motors have been shown in Figure 3 it is evident that any number of motors, within the capacity of the pump I, to sup:

ply fluid thereto, may be successfully operated and controlled by the system of this invention. It will be understood that the invention is not limited to the particular structure shown in the drawings, but also embraces such modifications as come within the scope of the claims.

What is claimed is: 1. In a hydraulic system, a plurality of fluid ciated motor; an adjustable restricting valve connected with the outlet of said variable restricting valve for passing the entire discharge therefrom; said variable restricting valve comprising a first fluid operable area connected with the outlet thereof and adapted for urging said valve into increased restricting position, and a second fluid operable area adapted for urging said'valve into decreased restricting position; a check valve connected with each of the outlets of said variable restrictors and opening outwardly therefrom into a common channel; means including a choke valve vhydraulically connecting said channel with all of said second fluid operable areas; and fluid pressure responsive fiow control means associated with said fluid source hydraulically connected with said second fluid operable areas whereby the speed of each motor is maintained substantially constant and independent of the speeds of the other of said motors and the delivery from said fluid source is varied in accordance speeds of said motors.

4. In a hydraulic system; a plurality of fluid operable motors having inlets and outlets; a fluid source for supplying fluid to said inlets; individual control means associated with said motor outmotors having their inlets connected together;

a variable restriction connected with theoutlet of each of said motors; an adjustable restriction connected with the outlet of each of said variable restrictions; each of said variable restrictions comprising a first and a second fluid operable means adapted for urging said restriction into increased or decreased restricting condition respec- V tively; means hydraulically connecting each of the first of said fluid operable means with the outlet of the associated restrictor; a common channel interconnecting all of said second fluid operable means; a check valve connected with the outlet of each of said variable restrictions and opening into said channel; and a fluid source connected to supply actuating fluid to said motor inlets and having associated therewith flow controlling means connected with said channel for varying the supply of fluid to said inlets inversely motors; opposed fluid operable means for varying said variable restrictor, one of the fluid operable means of each of said variable restrictions being connected with the outlet thereof and the other of said fluid operable means being connected together by a common channel; a check lets operable automatically to maintain the individual speeds of said motors substantially constant irrespective of the variations in loads thereon, each of said control means comprising a variable flow restricting means and a restrictor connected between a motor outlet and exhaust in the order named, fluid operable means responsive to the discharge pressure of each variable flow restricting means for increasing the restriction thereof, other fluid operable means for decreasing the restriction of said variable flow restricting means,

and means of selectively and continuously connecting said other fluid operable means with the inlet of the restrictor having the highest inlet pressure. v

5. In a fluid operable system; a plurality of fluid operable motors having inlets-and outlets; a fluid source of variable delivery connected to said inlets for supplying actuating fluid to said motors, yielding means associated with said source for continuously urging the same toward increased delivery position, fluid operable means associated with said fluid source for urging the same toward reduced delivery position; control means associated with the outlet of each of said motors, each of said control means comprising a variable choke valve having its inlet connected with the outlet of said motor and an adjustable I choke connected to the outlet of said variable valve connected with the outlet of each of said means; and fluid pressure responsive flow control means for varying the supply of fluid from said source to said motor inlets connected with said second fluid operable means.

3. In a hydraulic system; a fluid source of variable delivery; a plurality of fluid motors having their inlets connected for simultaneously rechoke valve, the outlet of said adjustable choke valve being connected with exhaust; a first fluid operable area associated with each of said variable choke valves for urging the same toward increased restriction position a second fluid operable area associated with each of said variable choke valves for urging the same toward decreased restriction position; means hydraulically connecting each of said first fluid operable areas with the outlet oi the associated valve; means hydraulically connecting all of said second fluid operable areas with the fluid operable means associated with said source; and means for automatically and continuously conducting fluid to said fluid operable means and to said second fluid operable areas only from the outlet of the variable choke valve having the greatest pressure.

with the I or lesser restriction for permitting the exhaust or fluid from said fluid pressure responsive means to a variable delivery pump connected with said in-" lets for supplying actuating fluid to said motors, fluid pressure responsive means associated with said pump and operable in response to a predetermined pressure to move said pump toward decreased delivery position; a flow control means associated with each of said motor outlets, each of said flow control means comprising a variable choke valve and an adjustable choke valve, said variable choke valve havingits inlet connected with the exhaust of the associated motor and its outlet connected to the inlet of said adjustable choke valve and the outlet of said adjustable choke valve being connected with exhaust; a first and a second opposed area associated with said variable choke valve for moving the same, respectively, into increased or decreased restriction p-' sition; means hydraulically connecting the flrst of said areas with the outlet of said variable choke valve; a check valve opening from each of said variable choke outlets into a common channel, a choke valve having its inlet connected to said channel and its outlet connected with said second areas and with said fluid pressure responsive means.

7. In a fluid operable system, a plurality of fluid operable motors having inlets and outlets, a variable delivery pump connected with said inlets for supplying actuating fluid to said motors, fluid pressure responsive means associated with germit said pump to move to full delivery posi- 8. In a. fluid operable system; a plurality of fluid operablemotors'having inlets and outlets, a fluid source for supplying fluid to said inlets;

individual flow control means connected with the outlet of each of said motors for controlling the speed of said motors and each comprising a variable flow restricting means connected with the motor outlet and a choke between the variable flow restricting means and exhaust; each of said variable flow restricting means being responsive to increases inmressure at its discharge side for moving to increased restriction position, and also being continuously responsive to the highest presfluid motors; each of said motors having a varisaid pump and operable in response to predetermined pressure to move said pump toward decreased delivery position; a flow control means associated with each or said motor outlets, each of said flow control means comprising a variable choke valve and-an adjustable choke valve, said variable choke valve having its inlet connected with the exhaust oi the associated. motor and its outlet connected to the inlet of said adjustable choke valve and the outlet of said adjustable choke valve being connected with exhaust; a first and a second opposed area associated'with said variable choke valve for moving the same, respectively, into increased or decreased restriction position; means hydraulically connecting the first of said areas with the outlet of said variable choke valve; a conduit connecting said second areas with said fluid pressure responsive means; check valves connected to open from each of the outlets of said variable choke valves into said conduit for supplying actuating fluid to said second areas and to said fluid pressure responsive means; and bypass means adapted for being made effective in response to the positioning of any of said variable choke valves in a, predetermined position able flow restricting means connected with its outlet, and each of said flow restricting means having a choke connected with its outlet; means responsive to the discharge pressure of each of said flow restricting means for increasing the effect thereof; means continuously and selectively responsive to the highest pressure at the inlet of any of said chokes for decreasing the efiects of said flow restricting means; fluid supply means for supplying actuating fluid to the inlets of said motors; and flow controlling means for said fluid supply means connected to be responsive to said last mentioned pressure.

' JQHAN A. MULLER.

WARREN R; TUCKER.

REFERENCES CITED 7 The following references are of record in the flle of this patent:

UNITED STATES PATENTS 549,211 British of 1942 

